Recently, zirconium phosphate-based inorganic ion exchangers have been used for a variety of applications based on the characteristics thereof. Zirconium phosphate-based inorganic ion exchangers include amorphous zirconium phosphates and crystalline zirconium phosphates including a 2-dimensional layered structure or a 3-dimensional network structure. Among these, hexagonal zirconium phosphates having a 3-dimensional network structure exhibit superior heat resistance, chemical resistance, radiation resistance and low thermal expansion properties and the like and are thus being applied to immobilization of radioactive wastes, solid electrolytes, gas adsorbing/separating agents, catalysts, raw materials for antimicrobial agents and the like.
Various hexagonal zirconium phosphates are currently known. For example, AxNH4(1+x)Zr2(PO4)3.nH2O is disclosed in PTL (Patent Literature) 1, AZr2(PO4)3.nH2O is disclosed in PTL 2, and HnR(1−n)Zr2(PO4)3.mH2O is disclosed in PTL 3.
In addition, zirconium phosphates having different ratios of Zr and P are also known. For example, Na1+4xZr2−x(PO4)3, is disclosed in NPL 1, and Na1+2xMgxZr2−x(PO4)3 is disclosed in NPL (Non-patent Literature) 1 and 2. Na1+xZr2SixP3−xO12 is disclosed in NPL 2 and 3.
Methods for synthesizing hexagonal zirconium phosphates include a high-temperature heating synthesis method in which synthesis is carried out by mixing solid-form powder raw materials and then heating the mixture at a high temperature of 1,000° C. or more using a heating furnace, a hydrothermal method in which synthesis is carried out by mixing raw materials dissolved in water or mixing raw materials in water and heating the water-containing materials under pressure, and a wet method in which synthesis is carried out by heating water-containing raw materials at normal pressure, or the like.
Among these, the high-temperature heating synthesis method is carried out simply by preparing raw materials and heating the same at a high temperature, thus enabling synthesis of zirconium phosphate with an appropriately adjusted P/Zr ratio. However, with the high-temperature heating synthesis method, it is not easy to uniformly mix raw materials and it is difficult to obtain zirconium phosphate with a homogeneous composition. In addition, in order to convert agglomerate-form zirconium phosphate obtained by the high-temperature heating synthesis method into a powder zirconium phosphate, pulverization and screening are required, thus causing problems associated with quality and production efficiency. In addition, it is natural that, with the high-temperature heating synthesis method, it is impossible to synthesize crystalline zirconium phosphate containing ammonia. On the other hand, with a hydrothermal or wet method, it is possible to obtain zirconium phosphate with homogeneous particulates.
Ions such as silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium have been known for a long time as metal ions that exhibit antimold properties, antimicrobial properties and antialgal properties (hereinafter, referred to as “antimicrobial metal ions”). However, silver ions are widely used as an aqueous silver nitrate solution which exhibits disinfection or pasteurization actions. However, metal ions which exhibit antimold, antimicrobial or antialgal properties are often harmful to humans, thus there are various restrictions on an application method, a storage method, a disposal method and the like as well as limitations on the applications.
However, antimold properties, antimicrobial or antialgal properties can be obtained by applying a small amount of antimicrobial metals to subjects. As a result, as antimicrobial agents which exhibit antimold, antimicrobial or antialgal properties, organic support antimicrobial agents obtained by supporting antimicrobial metal ions on an ion exchange resin, a chelate resin or the like, and inorganic antimicrobial agents obtained by supporting antimicrobial metal ions on a clay mineral, an inorganic ion exchanger or a porous body have been suggested.
Among these, silver-based inorganic antimicrobial agents obtained by supporting silver ions on an inorganic compound have properties such as improved safety, long-lasting antimicrobial effects and superior heat resistance as compared to a silver nitrate aqueous solution, thus having few restrictions on an application method, a storage method, a disposal method and applications as well as currently being applied to a variety of products. However, silver ions are unstable when exposed to heat and light and are immediately reduced into silver metals, thus having a problem of stability such as discoloration over a long period of time. Depending on the type of inorganic compounds to support the silver ions, the performance of the obtained silver-based inorganic antimicrobial agents varies and the antimicrobial agents frequently have restrictions.
As a silver-based inorganic antimicrobial agent, an antimicrobial agent obtained by ion exchanging alkali metal ions such as sodium ions in clay minerals such as montmorillonites and zeolites, with silver ions, is known. Since the skeleton structure of a clay mineral has deteriorated acid resistance, for example, silver ions are readily eluted in an acidic solution and the antimicrobial effects cannot last for a long time.
In addition, there are silver-supports obtained by adding silver ions and ammonium ions to zeolites by ion exchange in order to impart stability to silver ions. However, in this case, the prevention of discoloration does not reach a practical level or provide a fundamental solution.
There are antimicrobial agents in which antimicrobial metals are supported on adsorbent active carbons. However, since these agents physically adsorb or adhere soluble antimicrobial metal salts, on contact with water, antimicrobial metal ions are rapidly eluted and the duration of the antimicrobial effects thereof is thus lost.
Recently, an antimicrobial agent obtained by supporting antimicrobial metal ions on a specific zirconium phosphate has been suggested. For example, PTL 4 discloses the following Formula (2).M1aAbM2c(PO4)d.nH2O  (2)
(In Formula 2, M1 represents a metal ion selected from silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium and chromium, A represents at least one ion selected from an alkali metal ion, an alkaline earth metal ion, an ammonium ion and a hydrogen ion, M2 represents a tetravalent metal, n represents a number satisfying 0≦n≦6, a and b are positive numbers, c and d satisfy c=2 and d=3, provided that Ia+mb=1, and c and d satisfy c=1 and d=2, provided that la+mb=2, with the provisio that I is the valence of M1 and m is the valence of A.)
Such an antimicrobial agent is known as a material which is chemically and physically stable and exerts antimold properties and antimicrobial properties for a long period of time. However, the elution rate of silver ions increases under specific environmental conditions, for example, in cases of applications requiring exposure to water for a long period of time and some antimicrobial products cannot thus exert long-lasting effects for a long period of time.